The present invention relates to a method of analyzing a gas for trace impurities by atmospheric pressure ionization mass spectroscopy (APIMS). More particularly, the present invention relates to such a method in which moisture is removed from the gas prior to performing the analysis.
In ultra-high purity gas distribution systems it is necessary to ensure that the ultra-high purity gas retains its purity from the point of supply to the point of delivery. This is particularly important in the semiconductor industry because impurities present in an ultra-high purity gas being used in the manufacture of semiconductors can reduce yield of finished product.
In the semiconductor industry, APIMS has arisen as a principal technique in assuring the continued purity of ultra-high purity gases by being able to measure trace impurities in parts per billion and parts per trillion. The performance of an APIMS analysis is not however problem free in that moisture levels of as low as one part per billion can introduce substantial analytical errors when an ultra-high purity gas is being analyzed for the presence of trace impurities other than moisture. The errors arise from plasma saturation effects and from interference of moisture generated ions with signals of, for instance, trace levels of oxygen, methane and carbon dioxide ions.
In the prior art of moisture removal from gases materials such as alumina, organic solvents, sieving materials and etc. have been used. These techniques are not applicable to APIMS analysis because they either remove secondary trace impurities such as carbon dioxide and oxygen or they add impurities, such as organics. In order to overcome the problem of the presence of moisture in a gas sample to be analyzed by APIMS, the effect of moisture is compensated for in the results by numerical techniques. The problem with the use of numerical techniques is that they represent mathematical approximations and as such present an inherent margin of error.
As will be discussed, the present invention provides a method of performing an APIMS analysis in which moisture is removed from the gas sample to be analyzed before conducting the analysis so that numerical techniques do not have to be utilized in interpreting the results of the analysis.